The invention is based on a fuel injection arrangement for internal combustion engines, particularly for diesel engines.
In diesel engines with direct injection, considerable noise problems arise because of the sudden combustion. A noticeable reduction in noise can nevertheless be achieved by means of dividing the fuel injection quantity into a smaller advance injection quantity and a main injection quantity which depends on the speed and the load of the diesel engine. Therefore, in order to divide up the injection quantity a separate advance injection unit is provided in addition to the fuel injection pump, which separate advance injection unit feeds an advance injection quantity to the injection nozzle.
In a known fuel injection arrangement for combustion engines (DE-OS 27 21 628), the advance injection unit comprises an advance injection pump which is provided with a piezoelectric drive. The piezoelectric drive is excited by a control circuit which in turn receives a signal from a rotating part of the internal combustion engine, e.g. from the crankshaft, which signal is a function of the angle of rotation. The fuel injection pump, which has a conventional construction, is driven in synchronization with the internal combustion engine. The advance injection pump comprises a plunger which is axially displaceable in a cylinder which is connected with the fuel line between the fuel injection pump and the injection nozzle. When the piezoelectric drive is excited, it moves the plunger to the smallest cylinder volume, possibly by means of a hydraulic transmission. Accordingly, fuel is displaced from the cylinder and effects an opening of the injection nozzle through which the advance injection quantity exits At the end of the plunger stroke the piezoelectric drive remains excited so that the fuel delivered by the fuel injection pump now keeps the injection nozzle open or opens it again and the main injection quantity can accordingly exit through the opened injection nozzle. Toward the end of the delivery stroke of the fuel injection pump the piezoelectric drive is de-energized so that the plunger is moved back into the position with the greatest cylinder volume. The volume of the fuel line is accordingly increased in a sudden manner. The pressure in the fuel line and in the injection nozzle accordingly falls suddenly and closes the injection nozzle.
In this construction of the advance injection unit, the advance injection quantity is determined by means of the stroke of the plunger and fixed unalterably with respect to construction. Only the timing of the advance injection can be controlled. Accordingly, an optimum design of the advance injection with respect to maximum noise reduction is not possible because of the lack of adjustment possibilities. Moreover, the timing of the advance injection can also not be altered without influencing the main injection, since the latter is always effected immediately after the advance injection is terminated.